The present invention relates to automatic focusing apparatus, and more particularly to automatic focusing apparatus which uses two different types of focus detectors.
Apparatus for automatically focusing an image of an object formed by an optical system, such as a lens, are known and are used in various types of optical imaging equipment, such as self-focusing cameras. Such apparatus generally operate by detecting the degree of focus of the image 10 in the focal plane of the imaging system and by appropriately altering the focus of the optical system, usually with a servomotor, to maximize the degree of focus. One known technique for detecting focus is by monitoring with photosensors the relative displacement of two images of the same object formed by two optical systems through two different optical paths. The relative image displacement technique of focus detection is illustrated in FIGS. 1 and 2. Referring now to FIG. 1, there is shown schematically a focus detection system 100 based on the relative image displacement technique. The apparatus includes two optical imaging systems 107 and 108 each comprising a lens 109 and 110 for forming an image of an object 111 in a respective focal plane 112 and 113. Situated in the focal plane of each imaging system is an array of photosensors 101 and 102. Each photosensor in the array is adapted to sense the light intensity in a respective portion of the focal plane and to provide an electrical signal which is representative of the sensed light intensity. The signals from the photosensors in the two arrays 101 and 102 are provided to a relative image displacement detection circuit 114.
The image of the object 111 formed by the imaging system 107 is displaced relative to the one formed by the imaging system 108, owing to the differences between the optical paths 115 and 116 through which the images are respectively formed. The optical path 116 is coincident with the optical axis of the lens 110. As such, the image formed by the imaging system 108 is centered on the photosensor array 102. By contrast, the optical path 115 is at an angle with respect to the optical axis of the lens 109. Consequently, the image formed by the imaging system 107 is shifted to the left of the photosensor array 101. However, the angle between the optical path 115 and the optical axis of the lens 109 can be altered by means of a suitably shaped prism 117 in the optical path 115. Therefore, the relative displacement between images formed by the imaging systems 107 and 108 can be eliminated by an appropriate rotation of the prism 117. In the system of FIG. 1, the rotation of the prism 117 to eliminate the relative image displacement is accomplished by means of a servosystem 118 which receives a displacement signal from the image displacement detection circuit 114. The servosystem 118 provides the quantities g and h, which represent the sense and magnitude of rotation of the prism 117 required to eliminate the relative image displacement. The quantities g and h, which are derived from conventional encoding circuits within the servosystem 118, can in turn be used to alter the focus of an objective lens in the imaging equipment by means of another appropriate servosystem.
Turning now to FIG. 2, there is depicted a schematic diagram of the photosensor arrays 101 and 102 and the image displacement detection circuit 114 of the apparatus of FIG. 1. For purposes of illustration, the photosensor arrays 101 and 102 each comprise five photosensors d.sub.1 -d.sub.5 and d.sub.1 '-d.sub.5 ' respectively. The image 104 being sensed by the array 102 is shown as being centered in that array, while the image 103 being sensed by the array 101 is shown as being shifted towards one end of the array. The outputs of corresponding pairs of photosensors from each array are provided to respective differential amplifiers 119 -123. The outputs of the differential amplifiers 119 -123 are received by a summing circuit 106, which provides an output signal representative of the degree of relative displacement of the images sensed by the photosensor arrays 101 and 102.
The relative image displacement focus detection technique is advantageous in that it can be used to control automatic focusing apparatus even when the imaging equipment is far out of focus. However, the relative image displacement technique has the problem in that it lacks sufficient accuracy for fine focusing of the imaging equipment, particularly when the image has a repetitive pattern such as a striped or checkered pattern.
Another known technique for detecting focus in imaging equipment is to monitor the contrast of the image (i.e., the relative intensities of different portions of the image) with an array of photosensors arranged in the focal plane of the imaging equipment. According to the contrast detection technique, focus of an image is obtained when the contrast of the image, as detected by the photosensor array, is maximized. The contrast detection technique is advantageous in that it provides highly accurate focusing. However, this technique has the problem in that it cannot be used to control automatic focusing apparatus when the imaging equipment is far out of focus, since the contrast of an image falls off rapidly to zero as the image goes out of focus.
Therefore, a need exists for an automatic focusing apparatus which provides accurate focusing of imaging equipment, and yet can be used to control focusing even when the imaging equipment is far out of focus.